Optical system and optical film

ABSTRACT

Optical films, such as reflective polarizer films, and optical systems including the optical films are described. An optical system includes one or more optical lenses having at least one curved major surface, a partial reflector, and a reflective polarizer. For a substantially normally incident light in a predetermined wavelength range extending at least from about 450 nm to about 600 nm: the partial reflector has an average optical reflectance of at least 30%, and the reflective polarizer has an average optical reflectance Rs for a first polarization state, an average optical transmittance Tp for an orthogonal second polarization state, and an average optical reflectance Rp for the second polarization state, where Tp≥80%, Rp≤1%, and 50%≤Rs≤95%.

BACKGROUND

Optical systems can be used in head-mounted displays, for example, toprovide images to a viewer. The optical system can include an opticalfilm such as a reflective polarizer film.

SUMMARY

In some aspects of the present description an optical system includingone or more optical lenses having at least one curved major surface, apartial reflector, and a reflective polarizer is provide. For asubstantially normally incident light in a predetermined wavelengthrange extending at least from about 450 nm to about 600 nm: the partialreflector has an average optical reflectance of at least 30%; and thereflective polarizer has an average optical reflectance Rs for a firstpolarization state, an average optical transmittance Tp for anorthogonal second polarization state, and an average optical reflectanceRp for the second polarization state, where Tp≥80%, Rp≤1%, and50%≤Rs≤95%.

In some aspects of the present description an optical system includingone or more optical lenses, a partial reflector disposed on andconforming to a curved major surface of the one or more optical lenses;a reflective polarizer disposed on and conforming to a major surface ofthe one or more optical lenses and including a plurality of polymericlayers, each polymeric layer having an average thickness of less thanabout 500 nm; and an exit surface is provided. For a substantiallynormally incident light in a predetermined wavelength range extending atleast from about 450 nm to about 600 nm: the partial reflector has anaverage optical reflectance of at least 30%; and the plurality ofpolymeric layers has an average optical reflectance Rs for a firstpolarization state, where 50%≤Rs≤95%, and an average opticaltransmittance Tp≥80% for an orthogonal second polarization state. Theoptical system is configured to display an image to a viewer positionedproximate the exit surface. For an incident cone of light having a fullcone angle of at least 10 degrees that is incident on the optical systemfrom an object having a spatial frequency of less than about 1 line pairper millimeter and that exits the optical system through the exitsurface as an exiting cone of light, when the exiting cone of light isimaged proximate the exit surface, the image has a plurality ofalternating bright and dark regions. Ib is an average brightness ofcentral 50% regions of the bright regions, Id is an average brightnessof central 50% regions of the dark regions, and Ib/Id≥50.

In some aspects of the present description, an optical film including aplurality of alternating first and second polymeric layers numberingbetween 200 and 500 is provided. Each first and second polymeric layerhas an average thickness less than about 500 nm. For each pair ofadjacent first and second polymeric layers: the first layer has an indexn1x along a first axis in a plane of the optical film, an index ofrefraction n1y along an orthogonal second axis in the plane of theoptical film, and an index n1z along a z-axis orthogonal to the firstand second axes; and the second layer has an index n2x along the firstaxis, an index of refraction n2y along the second axis, and an index n2zalong the z-axis. For at least one wavelength in a predeterminedwavelength range extending at least from about 450 nm to about 600 nm: amaximum difference between n1x, n1y and n1z is less than about 0.002;and a difference between n2x and n1x is greater than about 0.2. For asubstantially normally incident light having the at least one wavelengthin the predetermined wavelength range, the plurality of alternatingfirst and second polymeric layers has an average optical reflectance Rsfor a first polarization state along the first axis, and an averageoptical transmittance Tp and an average optical reflectance Rp for asecond polarization state along the second axis, where Tp≥80%, Rp≤0.25%,and 80%≤Rs≤95%.

In some aspects of the present description, an optical system includingone or more optical lenses; a partial reflector disposed on andconforming to a curved major surface of the one or more optical lenses;a reflective polarizer disposed on and conforming to a major surface ofthe one or more optical lenses; and an exit surface is provided. Thereflective polarizer includes a plurality of alternating first andsecond polymeric layers, where each polymeric layer has an averagethickness of less than about 500 nm. For a substantially normallyincident light in a predetermined wavelength range extending at leastfrom about 450 nm to about 600 nm: the partial reflector has an averageoptical reflectance of at least 30%; and a maximum index of refractionof the second polymeric layer is greater than a maximum index ofrefraction of the first polymeric layer, and a difference between themaximum index of refraction of the second polymeric layer and a minimumindex of refraction of the first polymeric layer and is less than about0.3. The optical system configured to display an image to a viewerpositioned proximate the exit surface. For an incident cone of lighthaving a full cone angle of at least 10 degrees that is incident on theoptical system from an object comprising a spatial frequency of lessthan about 1 line pair per millimeter and that exits the optical systemthrough the exit surface as an exiting cone of light, when the exitingcone of light is imaged proximate the exit surface, the image has aplurality of alternating bright and dark regions. Ib is an averagebrightness of central 50% regions of the bright regions, Id is anaverage brightness of central 50% regions of the dark regions, andIb/Id≥50.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-2 are schematic cross-sectional views of optical systems;

FIG. 3 is a schematic cross-sectional view of an optical systemincluding first and second optical lenses;

FIG. 4 is a schematic cross-sectional view of an optical systemconfigured to display an image from a display panel to a viewer;

FIG. 5A is a schematic illustration of an object that can be displayedon a display panel;

FIG. 5B is a schematic illustration of an image formed from the objectof FIG. 5A;

FIG. 6 is a schematic cross-sectional view of an optical systemincluding a partial reflector facing a display panel;

FIG. 7 is a schematic cross-sectional view of an optical systemincluding a reflective polarizer facing a display panel;

FIG. 8 is a schematic plot of the transmittance of an optical film, orof a plurality of polymeric layers included in the optical film, as afunction of wavelength;

FIG. 9 is a schematic plot of the reflectance of an optical film, or ofa plurality of polymeric layers included in the optical film, as afunction of wavelength;

FIG. 10A is a schematic perspective view of an optical film;

FIG. 10B is a schematic perspective view of a segment of the opticalfilm of FIG. 10A;

FIGS. 11A-11B are schematic plots of retardance versus wavelength;

FIG. 12 is a schematic top view of a headset;

FIG. 13 is a plot of transmittance through reflective polarizers versuswavelength for the block and pass states; and

FIG. 14 is a plot of reflectance from reflective polarizers versuswavelength for the block and pass states.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanyingdrawings that form a part hereof and in which various embodiments areshown by way of illustration. The drawings are not necessarily to scale.It is to be understood that other embodiments are contemplated and maybe made without departing from the scope or spirit of the presentdescription. The following detailed description, therefore, is not to betaken in a limiting sense.

Folded optics systems such as those described in U.S. Pat. No. 9,557,568(Ouderkirk et al.), for example, utilize a reflective polarizer and apartial reflector to provide a folded optical path. Such optical systemscan be used in a head-mounted display, for example, to provide a highfield of view, for example, to a viewer. The reflective polarizer insuch optical systems have typically been chosen to provide a highreflection in a block state (e.g., a reflectance in the block state, Rs,of greater than 97%) and a high transmission in a pass state in order toprovide a high efficiency. As described in U.S. Pat. No. 9,557,568(Ouderkirk et al.) uniaxially oriented reflective polarizers, such asAPF available from 3M Company (St. Paul, Minn.), provide advantages whenused in a folded optics system. An APF film having single packet of 275interference layers had an internal average block state reflectance Rsof about 98% and an immersed average pass state reflectance, Rp, ofabout 2%. According to the present description, it has been found that areflective polarizer having a substantially lower pass state reflectance(e.g., a pass state reflectance, Rp, of no more than 1% or no more than0.6%) provides a higher contrast than using conventional reflectivepolarizers even when the reflectance in the block state is sacrificed(e.g., an Rs of no more than 95%) in order to provide the low pass statereflectance. It has been found that reflection from the reflectivepolarizer in the pass state causes a reduction in contrast due tomultiple subsequent reflections in the optical system of the reflectedpass state light.

It has been found that to achieve a desired low Rp, Rs is typicallyreduced. For example, in some embodiments, the reflective polarizer is amultilayer optical film including alternating layers of higher and lowerindex polymeric layers. Decreasing the number of layers or decreasingthe index difference between the higher and lower index layers in such areflective polarizer can decrease Rp (e.g., due to decreased reflectionsfrom mismatches in refractive indices along the pass direction) but alsodecrease Rs. As another example, in some embodiments, the reflectivepolarizer is a wire grid polarizer. Decreasing the wire density candecrease Rp but also decrease Rs.

FIG. 1 is schematic cross-sectional view of an optical system 100including one or more optical lenses 110 having at least one curvedmajor surface 112, a partial reflector 120, and a reflective polarizer130. The optical system 100 is configured such that for a substantiallynormally incident light 140 in a predetermined wavelength rangeextending at least from about 450 nm to about 600 nm: the partialreflector 120 has an average optical reflectance of at least 30%, andthe reflective polarizer 130 has an average optical reflectance Rs for afirst polarization state 142, an average optical transmittance Tp for anorthogonal second polarization state 144, and an average opticalreflectance Rp for the second polarization state 144. In someembodiments, Tp≥80%, Rp≤1%, and 50%≤Rs≤95%. In some embodiments, Tp≥85%or Tp≥90%. In some embodiments, Rp≤0.8%, or Rp≤0.6%, or Rp≤0.4%, orRp≤0.25%, or Rp≤0.2%. In some embodiments, 80%≤Rs≤95% or 85%≤Rs≤95%. Insome embodiments, for the substantially normally incident light 140 inthe predetermined wavelength range, the reflective polarizer has anaverage optical transmittance Ts of less than about 10% or less thanabout 5% for the first polarization state 142. Tp, Ts, Rp and Rs referto the unweighted average of the corresponding transmittance orreflectance over the predetermined wavelength range. In some cases, thecorresponding photopically averaged transmittance or reflectance T_(p)^(ph), T_(s) ^(ph), R_(p) ^(ph) and R_(s) ^(ph), respectively, may bespecified. T_(p) ^(ph), T_(s)ph, R_(p) ^(ph) and R_(s) ^(ph) may be inany of the respective ranges described for Tp, Ts, Rp and Rs. Thephotopic weighting used to determine T_(p) ^(ph), T_(s) ^(ph), R_(p)^(ph) and R_(s) ^(ph) may be defined by the CIE (InternationalCommission on Illumination) 1931 2° Standard Observer and the lightsource used in determining T_(p) ^(ph), T_(s) ^(ph), R_(p) ^(ph) andR_(s) ^(ph) may be the CIE Illuminant C.

The predetermined wavelength range extends from at least from about 450nm to about 600 nm. It has been found average reflectances and averagetransmittances over wavelength ranges extending at least from about 450nm to about 600 nm are useful in characterizing the reflectivepolarizer. The predetermined wavelength range may extend to wavelengthslower than 450 nm and/or to wavelengths higher than 600 nm. In someembodiments, the predetermined wavelength range extends at least fromabout 400 nm to about 600 nm, or extends at least from about 450 nm toabout 650 nm, or extends at least from about 450 nm to about 700 nm, orextends at least from about 400 nm to about 700 nm. In some embodiments,the predetermined wavelength range is from about 450 nm to about 600 nm,or from about 450 nm to about 650 nm, or from about 450 nm to about 700nm, or from about 400 nm to about 700 nm.

In the illustrated embodiment, the partial reflector 120 is disposed onand conforms to the curved major surface 112 of the one or more opticallenses 110. In some embodiments, the reflective polarizer 130 isdisposed on and conforms to a major surface of the one or more opticallenses 110. For example, the reflective polarizer 130 may be disposed(e.g., directly or indirectly through one or more optical layer and/orone or more adhesive layers) on a major surface 114 of the one or moreoptical lenses 110, which may be a substantially planar surface asillustrated or which may be a curved surface. In the illustratedembodiment, only one lens 110 is included, but it will be understoodthat optical system 100 can include more than one lens (e.g., two ormore, or three or more optical lenses). For example, the partialreflector 120 may be disposed on a major surface a first optical lensand the reflective polarizer 130 may be disposed on a major surface of adifferent second optical lens (see, e.g., FIG. 3).

In some embodiments, the optical system 100 includes an optical axis 150such that a light ray propagating along the optical axis 150 passesthrough the one or more optical lenses 110, the reflective polarizer130, and the partial reflector 120 without being substantiallyrefracted. In some embodiments, the one or more optical lens 110, thereflective polarizer 130, and the partial reflector 120 are centered onthe optical axis 150. Without being substantially refracted means thatthe angle between a light ray incident on a surface and a light raytransmitted through the surface is no more than 15 degrees. In someembodiments, an angle between the incident ray and the transmitted rayis less than 10 degrees, or less than 5 degrees, or less than 3 degrees,or less than 2 degrees. The one or more optical lens 110, the reflectivepolarizer 130, and the partial reflector 120 are disposed in opticalcommunication with one another. Optical communication as applied to twoobjects means that light can be transmitted from one to the other eitherdirectly or indirectly using optical methods (e.g., reflection,diffraction, refraction).

In some embodiments, the optical system 100 further includes a retarderdisposed between the partial reflector 120 and the reflective polarizer130. The retarder may be a separate layer between the lens 110 and thereflective polarizer 130, or may be disposed on the lens 110 or onanother lens in the optical system, or may be disposed on the reflectivepolarizer 130. The retarder may have a quarter wave retardance at one ormore wavelengths in the predetermined wavelength range.

The path of light 140 is schematically illustrated in FIG. 1. The actualpath may be different from that illustrated (e.g., the illustratedspacing between different portions of the light path may not be toscale). A portion 140 a of the light 140 is transmitted through partialreflector 120 and is incident on the reflective polarizer in the blockstate 142. A portion 140 c (e.g., proportional to Rs) of the portion 140a is reflected from the reflective polarizer 130 and another portion 140b (e.g., proportional to Ts) is transmitted through the reflectivepolarizer. A portion 140 d of the portion 140 c is reflected from thepartial reflector 120 and another portion (not illustrated) istransmitted through the partial reflector 120. The portion 140 d isincident on the reflective polarizer 130 in the pass state 144. Aretarder can be included between the partial reflector 120 and thereflective polarizer 130 so that the portion 140 d is in the pass state144 when it is incident on the reflective polarizer 130. A portion 140 e(e.g., proportional to Tp) of the portion 140 d is transmitted throughthe reflective polarizer 130 and another portion 140 f (e.g.,proportional to Rp) is reflected from the reflective polarizer 130. Insome embodiments where light 140 is from a display panel and the opticalsystem 100 is configured to display the image to a viewer, the desiredimage is in portion 140 e. A portion 140 g of the portion 140 f isreflected from partial reflector 120 and another portion (notillustrated) is transmitted through the partial reflector 120. A portion140 h (e.g., proportional to Rs) of the portion 140 g is reflected fromreflective polarizer 130 and another portion (e.g., proportional to Ts;not illustrated) is transmitted through the reflective polarizer 130. Aportion 140 i of the portion 140 h is reflected from partial reflector120 and another portion (not illustrated) is transmitted through thepartial reflector 120. A portion 140 j (e.g., proportional to Tp) of theportion 140 i is reflected from reflective polarizer 130 and anotherportion (not illustrated) is reflected from the reflective polarizer130.

Undesired reflections can degrade the contrast of the optical system100. It has found that the portion 140 j has a large impact on theperceived contrast (e.g., as quantified by Ib/Id as described elsewhereherein) of the optical system 100 even though it is a higher orderreflection. The portion 140 b can be blocked by including a clean-uppolarizer (e.g., an optically absorptive polarizer between thereflective polarizer 130 and an exit surface of the optical system 100),but the portion 140 j has the same polarization state 144 as the portion140 e and so a clean-up polarizer would not eliminate portion 140 jwithout also attenuating portion 140 e. Other unwanted reflections notnecessarily shown in FIG. 1 can also reduce contrast. As describedfurther elsewhere herein (see, e.g., FIG. 6) a variety of index matchinglayers and/or antireflection coatings can be included to reduce unwantedreflections.

A display panel can be included to provide the light 140. Polarizingelements, such as an optically absorptive polarizer and a retarder, canbe included in the display panel or disposed between the display paneland the partial reflector 120 so that light 140 is substantially in thefirst polarization state 142 when first incident on the reflectivepolarizer 130.

The partial reflector used in any of the optical systems of the presentdescription may be any suitable partial reflector. For example, thepartial reflector may be constructed by coating a thin layer of a metal(e.g., silver or aluminum) on a transparent substrate (e.g., a filmwhich may then be adhered to a lens, or the substrate may be a lens).The partial reflector may also be formed by depositing thin-filmdielectric coatings onto a surface of a lens substrate, or by depositinga combination of metallic and dielectric coatings on the surface, forexample. In some embodiments, for a substantially normally incidentlight in a predetermined wavelength range extending at least from about450 nm to about 600 nm, the partial reflector has an average opticalreflectance of at least 30%. In some embodiments, the average opticalreflectance of the partial reflector is in a range of 40% to 60%. Insome embodiments, the partial reflector has an average opticalreflectance and an average optical transmittance in the predeterminedwavelength range that are each in a range 30% to 70%, or each in a rangeof 40% to 60%, or each in a range of 45% to 55%. The partial reflectormay be a half mirror, for example.

The average optical reflectance of the partial reflector refers to theoptical reflectance for substantially unpolarized light substantiallynormally incident on the partial reflector and averaged (unweightedmean) over wavelengths in the predetermined wavelength range, unlessspecified differently. The average optical reflectance and averageoptical transmittance for the reflective polarizer for a specifiedpolarization state refers to the optical reflectance and opticaltransmittance, respectively, for light substantially normally incidenton the reflective polarizer in the specified polarization state andaveraged (unweighted mean) over wavelengths in the predeterminedwavelength range, unless specified differently.

Substantially unpolarized light is light having a sufficiently smalldegree of polarization that the transmittance and reflectance ofnormally incident substantially unpolarized light differs negligiblyfrom that of normally incident unpolarized light. The degree ofpolarization is the fraction of light (by intensity) that is polarized.In some embodiments, light described as substantially unpolarized has adegree of polarization of less than 10%. In some embodiments, lightdescribed as substantially unpolarized is unpolarized or nominallyunpolarized. Substantially normally incident light is light sufficientlyclose to normally incident that the transmittance and reflectance ofsubstantially normally incident unpolarized light differs negligiblyfrom that of normally incident unpolarized light. Substantially normallyincident light may, in some embodiments, be within 20 degrees, or within10 degrees of normally incident, or may be normally incident ornominally normally incident.

The reflective polarizer may be any suitable reflective polarizer havingthe desired reflectivity properties. For example, a wire grid polarizerhaving a density of wires (e.g., nanowires) selected to provide Tp≥80%,Rp≤1%, and 50%≤Rs≤95% may be used. In some embodiments, the reflectivepolarizer includes a plurality of alternating first and second polymericlayers and the refractive indices of the first and second polymericlayers and the total number of the first and second polymeric layers areselected to provide Tp≥80%, Rp≤1%, and 50%≤Rs≤95% as described furtherelsewhere herein.

FIG. 2 is schematic cross-sectional view of an optical system 200including one or more optical lenses 210 having at least one curvedmajor surface 212, a partial reflector 220; and a reflective polarizer230. The optical system 200 is in many ways similar to optical system100 except for the relative orientations of the partial reflector andreflective polarizer. Optical system 200 is configured to receive lightincident on the reflective polarizer 230 and transmit light through thepartial reflector 220 to a viewer, while optical system 100 isconfigured to receive light incident on the partial reflector 120 andtransmit light through the reflective polarizer 130 to a viewer. Thereflective polarizer 230 may be as described for reflective polarizer130 and the partial reflector 220 may be as described for partialreflector 120. The optical system 200 may further include a retarderdisposed between the partial reflector 220 and the reflective polarizer230 as described for optical system 100. In the illustrated embodiment,only one lens 210 is illustrated, but it will be understood that opticalsystem 200 can include more than one lens (e.g., two or more, or threeor more optical lenses) as describe further elsewhere herein. Forexample, the reflective polarizer 230 may be disposed on a major surfacea first optical lens and the partial reflector 220 may be disposed on amajor surface of a different second optical lens (see, e.g., FIG. 3).

The path of light 240 having a pass (second) polarization state 244 isschematically illustrated in FIG. 2. The actual path may be differentfrom that illustrated (e.g., the illustrated spacing between differentportions of the light path may not be to scale). A portion 240 a (e.g.,proportional to Tp) of the light 240 is transmitted through thereflective polarizer 230 and another portion 240 b (e.g., proportionalto Rp) is reflected from the reflective polarizer 230. A portion 240 cis transmitted through the partial reflector 220 and another portion 240d is reflected from the partial reflector 220. The portion 240 d isincident on the reflective polarizer 230 in the block (first)polarization state 242. A portion 240 e (e.g., proportional to Rs) ofthe portion 240 d is reflected from the reflective polarizer 230 andanother portion (not illustrated) is transmitted through the reflectivepolarizer 230. A portion 240 f of the portion 240 e is transmittedthrough the partial reflector 220 and another portion (not illustrated)is reflected from the partial reflector 220. In some embodiments wherelight 240 is from a display panel and the optical system 200 isconfigured to display the image to a viewer, the desired image is inportion 240 f. The portion 240 b (and also portions of portion 240 d andof portion 240 i) transmitted through the reflective polarizer 230 isreflected from an object (not illustrated) in front of the opticalsystem 200 as portion 240 g. A portion 240 h (e.g., proportional to Tp)of portion 240 g is transmitted through reflective polarizer 230 asportion 240 h and another portion (not illustrated) is reflected fromreflective polarizer 230. A portion 240 i of portion 240 h is reflectedfrom the partial reflector 220 and another portion (not illustrated) istransmitted through the partial reflector 220. A portion 240 j (e.g.,proportional to Rs) is reflected from reflective polarizer 230 andanother portion (not illustrated) is transmitted through reflectivepolarizer 230. A portion 240 k of portion 240 j is transmitted throughpartial reflector 220 and another portion (not illustrated) is reflectedfrom partial reflector 220.

Undesired reflections can degrade the contrast of the optical system200. It has found that the portion 240 k has a large impact on theperceived contrast of the optical system 200 even though it is a higherorder reflection. The portion 240 c can be blocked by including aclean-up polarizer (e.g., an optically absorptive polarizer between thepartial reflector 220 and an exit surface of the optical system 200),but the portion 240 k has the same polarization state as the portion 240f and so a clean-up polarizer would not eliminate portion 240 k withoutalso attenuating portion 240 f. As described further elsewhere herein(see, e.g., FIG. 7) a variety of index matching layers and/orantireflection coatings can be included to reduce unwanted reflections.

A display panel can be included to provide the light 240. Polarizingelements, such as an optically absorptive polarizer, can be included inthe display panel or disposed between the display panel and thereflective polarizer 230 so that light 240 is substantially in thesecond polarization state 244 when first incident on the reflectivepolarizer 230.

FIG. 3 is a schematic cross-sectional view of an optical system 300including one or more optical lenses 310. In the illustrated embodiment,the one or more optical lenses 310 includes a first optical lens 311 anda second optical lens 313. An optical layer 315 is disposed on andconforms to a major surface of the first optical lens 311, which is acurved major surface of the first optical lens 311 in the illustratedembodiment. An optical layer 317 is disposed on and conforms to a majorsurface of the second optical lens 313, which is a curved major surfaceof the second optical lens 313 in the illustrated embodiment. Either orboth of the optical layers 315 and 317 may be films or coatings disposedon the respective first and second optical lenses 311 and 313. In someembodiments, one of the optical layers 315 and 317 is a reflectivepolarizer (e.g., corresponding to reflective polarizer 130 or 230),which may be a multilayer optical film, and the other of the opticallayers 315 and 317 is a partial reflector (e.g., corresponding topartial reflector 120 or 220). In some embodiments, the optical system300 has an optical axis 350 such that a light ray 340 propagating alongthe optical axis 350 passes through the one or more optical lenses 310,and the first and second optical layers 315 and 317 without beingsubstantially refracted. In some embodiments, the one or more opticallenses 310 are centered on the optical axis 350.

FIG. 4 is a schematic cross-sectional view of an optical system 400which includes an optical system 401 and a display panel 405. Opticalsystem 401 may correspond to optical system 100 or 200 or 300, forexample, and optical system 400 may correspond to optical system 600 or700 described elsewhere herein, for example. In some embodiments,optical system 401 includes one or more optical lenses; a partialreflector disposed on and conforming to a curved major surface of theone or more optical lenses; a reflective polarizer disposed on andconforming to a major surface of the one or more optical lenses andcomprising a plurality of polymeric layers, each polymeric layer havingan average thickness of less than about 500 nm, such that for asubstantially normally incident light in a predetermined wavelengthrange extending at least from about 450 nm to about 600 nm: the partialreflector has an average optical reflectance of at least 30%; and theplurality of polymeric layers has an average optical reflectance Rs fora first polarization state, 50%≤Rs≤95%, and an average opticaltransmittance Tp≥80% for an orthogonal second polarization state. Insome embodiments, Tp≥80%, Rp≤1%, and 50%≤Rs≤95%. In some embodiments,Tp≥85% or Tp≥90%. In some embodiments, Rp≤0.8%, or Rp≤0.6%, or Rp≤0.4%,or Rp≤0.25%, or Rp≤0.2%. In some embodiments, 80%≤Rs≤95% or 85%≤Rs≤95%.In some embodiments, for a substantially normally incident light in thepredetermined wavelength range, the plurality of polymeric layers has anaverage optical transmittance Ts of less than about 10% or less thanabout 5% for the first polarization state. The symbols Rs, Rp, Ts, andTp may be used to refer to the reflectance for the first and secondpolarization states and the transmittance for the first and secondpolarization states, respectively, for the reflective polarizer or forthe plurality of polymeric layers which may be a plurality ofinterference layers reflecting and transmitting light primarily byoptical interference. The values for the reflective polarizer may differslightly from the corresponding values for the plurality of polymericlayers dues to additional interfaces in the reflective polarizer. Insome embodiments, the reflective polarizer and the plurality ofpolymeric layers have Rs, Rp, Ts, and/or Tp in any of the above ranges.

Optical system 401 includes an exit surface 452 which is a surfacethrough which light incident on the optical system 401 (e.g., fromdisplay panel 405) exits the optical system 401. In some embodiments,exit surface 405 is the major surface of the component of the opticalsystem 401 farthest from the display panel 405. In some embodiments,exit surface 405 is not a surface of a component, but is a surfaceadapted to overlap an entrance pupil of a second optical system (e.g., acamera or a viewer's eye). The optical systems 400 and 401 areconfigured to display an image to a viewer 454 positioned proximate theexit surface 452.

A measure of the contrast of an optical system can be obtained bydisplaying an object having light and dark regions through the opticalsystem, forming an image of the object, and determining a ratio ofaverage brightness of bright regions to average brightness of darkregions in the image. In some embodiments, it is desired to define thecontrast for an object having a low spatial frequency (e.g., less thanabout 1 line pair per millimeter). In some case, some portions of thebright and dark regions near transition regions between bright and darkregions are excluded when forming the ratio characterizing the contrastsince such transition regions can have a high effective spatialfrequency and it may be desired to define the contrast ratio in thelimit of low special frequency. The object may include alternating lightand dark lines or light and dark squares or rectangles, for example. Theobject can be characterized in terms of line pairs (pairs of light anddark lines or regions) per millimeter. The average brightness can bedetermined by using a camera to measure the intensity and then averagingthe intensity over a specified area of bright regions (e.g., centralregions of the bright regions where each central region has an area ofabout 50% of the area of the corresponding bright region) and over aspecified area of dark regions (e.g., central regions of the darkregions where each central region has an area of about 50% of the areaof the corresponding dark region). The average brightness refers to theunweighted mean intensity unless indicated differently. A spatialfrequency of less than 1 line pair per millimeter may be used indetermining a measure of the contrast of the optical system, while otherspatial frequencies may be utilized when the optical system is used indisplay applications, for example. For example, on use, the opticalsystem may display an image of an object having a spatial frequencysubstantially greater than 1 line pair per millimeter (e.g., a highdefinition image).

In some embodiments, for an incident cone of light 440 having a fullcone angle θ (full width at half maximum) of at least 10 degrees (e.g.,10 degrees to 60 degrees, or 15 degrees to 40 degrees, or 20 degrees to35 degrees, or about 30 degrees) that is incident on the optical system401 from an object 407 (e.g., displayed on display panel 405) having aspatial frequency of less than about 1 line pair per millimeter (e.g.,about 0.1 to about 1 line pairs per millimeter), and that exits theoptical system 401 through the exit surface 452 as an exiting cone oflight 441, when the exiting cone of light 441 is imaged proximate theexit surface 452, the image has a plurality of alternating bright anddark regions. For example, FIG. 5A is a schematic illustration of anobject 507 that can be displayed on display panel 405. Object 507 mayextend over only a portion of the display panel. For example, a 14squares×14 squares checkerboard pattern can be displayed on the displaypanel and a portion of this pattern (e.g., 10 squares×10 squares, or 6squares×6 squares, or 5 squares×5 squares, or 4 squares×4 squares)checkerboard pattern near the center of the display panel) can be takento be the object 507. Object 507 has a first spatial frequency of 1 linepair per unit distance d1 along a first direction (x-direction) and asecond spatial frequency of 1 line pair per unit distance d2 along anorthogonal second direction (y-direction). The object 507 has a spatialfrequency of less than about 1 line pair per millimeter if at least oneof the first and second spatial frequencies is less than about 1 linepair per millimeter. In some embodiments, each of the first and secondspatial frequencies is less than about 1 line pairs per millimeter. Insome embodiments, Ib/Id≥50, where Ib is an average brightness of central50% regions of the bright regions, and Id is an average brightness ofcentral 50% regions of the dark regions. FIG. 5B is a schematicillustration of an image 509 formed light from the object 507 that hasexited the optical system 401 through the exit surface 452. The image509 may appear as the object 507 except for an overall scale or theimage 509 may be distorted by the optical system (such distortion, ifany, can be electronically corrected if desired). The image 509 has aplurality of alternating bright and dark regions 571 and 573. For eachbright region 571, there is a central 50% region 576, and for each darkregion 573, there is a central 50% region 579. The central 50% region576 refers to a region in an interior region of a bright region 571having an area of about 50% of the area of the bright region 571.Similarly, the central 50% region 579 refers to a region in an interiorof a dark region 573 having an area of about 50% of the area of the darkregion 573. The average brightness of central 50% regions 576 of thebright regions 571 is determined by averaging the brightness (intensity)over the central 50% regions 576. Similarly, the average brightness ofcentral 50% regions 579 of the dark regions 573 is determined byaveraging the brightness over the central 50% regions 579. The ratioIb/Id can be measured separately for red, green and blue subpixels; orcan be determined for a white light output; or can be determined for oneor more wavelengths in the predetermined wavelength range, for example.A full cone angle θ of at least 10 degrees may be used in determining ameasure of the contrast of the optical system, while other cone anglesmay be utilized when the optical system is used in display applications,for example. For example, in use, the optical system may include adisplay panel providing an incident cone of light having any suitablefull cone angle (e.g., less than 10 degrees, about 10 degrees, orgreater than 10 degrees).

In some cases, instead of using a display panel to display the object407 or 507, the object 407 or 507 is produced by using a light emittingdiode (LED) light source, for example, to illuminate a checkerboardpattern coated on a glass, for example (such patterns coated on glassare available from Precision Optical Imaging (Rush, N.Y.), for example).Optical components may be included between the light source and theglass to produce a specified full cone angle θ. The LED may be a whiteLED and Id and Ib may be determined using a color camera in any one ormore of the red, green, or blue color channels of the color camera, forexample. Similar or substantially the same results for Ib/Id can beobtained by using a light source in a wavelength range corresponding tothe wavelength range where the color channel of a color camera issensitive and determining the brightness over the entire wavelengthrange of the light emitted by the light source. For example, utilizing awhite light source and a green color channel of a color camera will givesimilar results for Ib/Id as using a light source having a wavelengthdistribution corresponding to the wavelengths of the green color channelof the color camera and determining the brightness over the entirewavelength range of the emitted light. In some embodiments, the incidentcone of light used in determining Ib/Id includes wavelengths from atleast 520 nm to 570 nm, and/or from at least 430 nm to 480 nm, and/orfrom at least 610 nm to 660 nm. In some embodiments, the light sourceemits light in the predetermined wavelength range.

In some embodiments, index matching layers, antireflection coatings,additional retarders, and/or optically absorptive polarizer(s) may beincluded in the optical system in order to reduce undesired reflectionsand thereby increase Ib/Id, for example. Any of the optical systems ofthe present description may have Ib/Id≥50, Ib/Id≥55, or Ib/Id≥60, orIb/Id≥65, Ib/Id≥70, or Ib/Id≥72, or Ib/Id≥75, or Ib/Id≥76, or b/Id≥77,or Ib/Id≥78, for example. In some embodiments, the incident cone oflight used to determine the Ib/Id in any of these ranges includes atleast one wavelength in the predetermined wavelength range. In someembodiments, the incident cone of light used to determine the Ib/Id inany of these ranges includes wavelengths from at least 520 nm to 570 nm,and/or from at least 430 nm to 480 nm, and/or from at least 610 nm to660 nm. In some embodiments, the incident cone of light includeswavelengths from at least 500 nm to 570 nm; and Ib/Id≥72, or Ib/Id≥75,or Ib/Id≥76, or Ib/Id≥77, or b/Id≥78. FIGS. 6-7 schematically illustratetwo optical systems each including one or more index matching layers andone or more antireflection coatings. In some embodiments, one or more ofthe index matching layers, antireflection coatings, additionalretarders, and/or optically absorptive polarizer(s) are omitted. In someembodiments, additional index matching layers, antireflection coatings,additional retarders, and/or optically absorptive polarizer(s) areincluded.

FIG. 6 is a schematic illustration of an optical system 600 includingone or more optical lenses 610, a partial reflector 620, and areflective polarizer 630. In some embodiments, the partial reflector 620is disposed on and conforms to a curved major surface 612 of the one ormore optical lenses 610 and the reflective polarizer 630 is disposed onand conforms to a major surface 614 of the one or more optical lenses610. In the illustrated embodiment, the partial reflector 620 isdisposed directly on the major surface 612 of the one or more opticallenses 610 and the reflective polarizer 630 is disposed indirectly onthe major surface 614 of the one or more optical lenses 610. In theillustrated embodiment, only one optical lens 611 in the one or moreoptical lenses 610 is illustrated. In other embodiments, two or moreoptical lenses are included as described further elsewhere herein.

In some embodiments, the optical system 600 is configured to display animage (e.g., from display panel 605) to a viewer positioned proximate anexit surface 652 of the optical system 600. The reflective polarizer 630is disposed between the partial reflector 620 and the exit surface 652.A retarder 625 is disposed between the partial reflector 620 and thereflective polarizer 630.

In the illustrated embodiment, the optical system 600 includes anintegral optical stack 690. An integral optical stack is a stack ofoptical layers or components that are bonded to one another. Theintegral optical stack 690 includes, in sequence, the partial reflector620, a first optical lens 611 in the one or more optical lenses 610, afirst optically clear adhesive layer 655, the retarder 625, a secondoptically clear adhesive layer 657, and the reflective polarizer 630. Insome embodiments, for at least one pair of adjacent layers in theintegral optical stack 690, an index matching layer is disposed betweenthe pair of adjacent layers. For example, in the illustrated embodiment,the integral optical stack 690 also includes a first index matchinglayer 624 disposed between the first optically clear adhesive layer 655and the retarder 625, a second index matching layer 626 disposed betweenthe retarder 625 and the second optically clear adhesive layer 657, anda third index matching layer 627 disposed between the second opticallyclear adhesive layer 657 and the reflective polarizer 630. In someembodiments, the integral optical stack 690 further comprises anantireflection coating 629 on a major surface of the reflectivepolarizer 630 opposite the second optically clear adhesive layer 657.

In some embodiments, the optical system 600 includes an opticallyabsorptive polarizer 661 disposed between the exit surface 652 and thereflective polarizer 630. Such an optically absorptive polarizer can beused as a clean-up polarizer. In some embodiments, the opticallyabsorptive polarizer 661 has an average optical absorption of greaterthan about 50%, or greater than about 60%, or greater than about 70%, orgreater than about 80%, for the first (block) polarization state in thepredetermined wavelength range. In some embodiments, the optical system600 includes a retarder 663 disposed between the optically absorptivepolarizer 661 and the exit surface 652. The retarder 663 may be includedso that reflections back into the optical system (e.g., from a viewer'seye) are rotated substantially to the block state of the opticallyabsorptive polarizer 661 after passing through the retarder 663 whenexiting and when reentering the optical system 600.

In some embodiments, the optical system 600 includes a display panel 605having an outermost major surface 662 facing the one or more opticallenses. In some embodiments, the outermost major surface 662 of thedisplay panel 605 includes an antireflection coating 649. Theantireflection coating 649 can be included to prevent light reflectedfrom the partial reflector 620 towards the display panel 605 from beingreflected back towards the partial reflector 620. In some embodiments,the display panel 605 includes at least one of a retarder 665 and anoptically absorptive polarizer 691, where the antireflection coating 649of the display panel 605 is disposed on the retarder 665 or theabsorbing polarizer 691. In the illustrated embodiment, a retarder 665and an optically absorptive polarizer 691 are included where theantireflection coating 649 is disposed on the retarder 665. In someembodiments, an index matching layer 647 is disposed between theoptically absorptive polarizer 691 and the retarder 665. In theillustrated embodiment, display panel 605 includes, in sequence, a firstdisplay panel 604, a first optically clear adhesive layer 675, aretarder 685, a second optically clear adhesive layer 677, the opticallyabsorptive polarizer 691, a third optically clear adhesive layer 678,the index matching layer 647, the retarder 665, and the antireflectioncoating 649. Additional index matching layers (e.g., on each majorsurface of the retarder 685) may also be included. The first displaypanel 604 may be configured to emit unpolarized light and the retarder685, second optically clear adhesive layer 677, and the opticallyabsorptive polarizer 691 may together be a circular polarizer (e.g., acircular polarizer as is commonly included in an organic light emittingdiode (OLED) display). The optically absorptive polarizer 691 and theretarder 665 can be included so that light is incident on the partialreflector 620 in a circularly polarized state and then is incident onthe reflective polarizer 630 after passing through the retarder 625 in ablock state of the reflective polarizer 630.

FIG. 7 is a schematic illustration of an optical system 700 includingone or more optical lenses 710, a partial reflector 720, and areflective polarizer 730. In some embodiments, the reflective polarizer730 is disposed on and conforms to a curved major surface 712 of the oneor more optical lenses 710 and the partial reflector 720 is disposed onand conforms to a major surface 714 of the one or more optical lenses710. In the illustrated embodiment, the reflective polarizer 730 isdisposed directly on the major surface 712 of the one or more opticallenses 710 and the partial reflector 720 is disposed indirectly on themajor surface 714 of the one or more optical lenses 710. In theillustrated embodiment, only one optical lens 711 in the one or moreoptical lenses 710 is illustrated. In other embodiments, more than oneoptical lens 711 is included as described further elsewhere herein.

In some embodiments, the optical system 700 is configured to display animage (e.g., from display panel 705) to a viewer positioned proximate anexit surface 752 of the optical system 700. The partial reflector 720 isdisposed between the reflective polarizer 730 and the exit surface 752.A retarder 725 is disposed between the reflective polarizer 730 and thepartial reflector 720. The reflective polarizer 730 may be an opticalfilm reflective polarizer as described further elsewhere herein.

In some embodiments, as described further elsewhere herein, thereflective polarizer 630 or 730 includes a plurality of polymeric layerswhere each polymeric layer having an average thickness of less thanabout 500 nm. In some embodiments, for a substantially normally incidentlight in a predetermined wavelength range extending at least from about450 nm to about 600 nm: the partial reflector 620 or 720 has an averageoptical reflectance of at least 30%, and the plurality of polymericlayers has an average optical reflectance Rs for a first polarizationstate, 50%≤Rs≤95%, and an average optical transmittance Tp≥80% for anorthogonal second polarization state. In some embodiments, Tp≥80%,Rp≤1%, and 50%≤Rs≤95%. In some embodiments, Tp≥85% or Tp≥90%. In someembodiments, Rp≤0.8%, or Rp≤0.6%, or Rp≤0.4%, or Rp≤0.25%, or Rp≤0.2%.In some embodiments, 80%≤Rs≤95% or 85%≤Rs≤95%. In some embodiments, fora substantially normally incident light in the predetermined wavelengthrange, the plurality of polymeric layers has an average opticaltransmittance Ts of less than about 10%, or less than about 5% for thefirst polarization state. In some embodiments, as described furtherelsewhere herein, for an incident cone of light having a full cone angleof at least 10 degrees that is incident on the optical system 600 or 700from an object comprising a spatial frequency of less than about 1 linepair per millimeter, and exits the optical system 600 or 700 through theexit surface 652 or 752 as an exiting cone of light, when the exitingcone of light is imaged proximate the exit surface 652 or 752, the imagehas a plurality of alternating bright and dark regions. In someembodiments, Ib/Id≥50 where Ib is an average brightness of central 50%regions of the bright regions, and Id is an average brightness ofcentral 50% regions of the dark regions. In some embodiments, Ib/Id≥55,Ib/Id≥60, or Ib/Id≥65, Ib/Id≥70, or Ib/Id≥72, or Ib/Id≥75, or Ib/Id≥76,or Ib/Id≥78. In some embodiments, each central 50% region of the brightregions is an interior region of a bright region having an area of about50% of an area of the bright region, and each central 50% region of thedark regions is an interior region of a dark region having an area ofabout 50% of an area of the dark region.

In the illustrated embodiment, the optical system 700 includes anintegral optical stack 790. The integral optical stack includes, insequence, the reflective polarizer 730, a first optical lens 711 in theone or more optical lenses 710, a first optically clear adhesive layer755, a retarder 725, and the partial reflector 720. In some embodiments,the integral optical stack 790 further includes, in sequence, a secondoptically clear adhesive layer 757 disposed on the partial reflector 720opposite the retarder 725, a second retarder 735, a third opticallyclear adhesive layer 759, an optically absorptive polarizer 761 whichmay have an average optical absorption of greater than about 50% for thefirst polarization state in the predetermined wavelength range, and athird retarder 763. In some embodiments, the integral optical stack 790further includes a fourth optically clear adhesive layer 779 between theoptically absorptive polarizer 761 and the third retarder 763. In someembodiments, for at least one pair of adjacent layers in the integraloptical stack 790, an index matching layer is disposed between the pairof adjacent layers. For example, in the illustrated embodiment, an indexmatching layer 724 is disposed between the first optically clearadhesive layer 755 and the retarder 725, an index matching layer 727 isdisposed between the second optically clear adhesive layer 757 and theretarder 735, an index matching layer 729 is disposed between theretarder 735 and the third optically clear adhesive layer 759, and anindex matching layer 769 is disposed between the fourth optically clearadhesive layer 779 and the third retarder 763. In some embodiments, theintegral optical stack has outermost first and second major surfaces 797and 798, and each of the outermost first and second major surfaces 797and 798 has an antireflection coating 795 and 796, respectively. Theretarder 763 may be included so that reflections back into the opticalsystem (e.g., from a viewer's eye) are rotated substantially to theblock state of the optically absorptive polarizer 761 after passingthrough the retarder 763 when exiting and when reentering the opticalsystem 700.

Optical system 700 includes a display panel 705 having an outermostmajor surface 762 facing the one or more optical lenses 710. In someembodiments, the outermost major surface 762 of the display panel 705includes an antireflection coating 749. In some embodiments, the displaypanel 705 includes at least one of a retarder 785 or an opticallyabsorptive polarizer 791, and the antireflection coating 749 of thedisplay panel 705 is disposed on the retarder 785 or the opticallyabsorptive polarizer 791. In the illustrated embodiment, the displaypanel 705 includes, in sequence, a first display panel 704, a firstoptically clear adhesive layer 775, an index matching layer 774, aretarder 785, an index matching layer 747, a second optically clearadhesive layer 778, an optically absorptive polarizer 791, and anantireflection coating 749 at the outermost major surface 762. Theoptically absorptive polarizer 791 may be included so that light fromthe display panel 705 is incident on the reflective polarizer 730 in thepass polarization state of the reflective polarizer 730.

Any suitable index matching layer or antireflection coating may be usedin any of the optical systems. An index matching layer between twoadjacent layers or components typically has at least one refractiveindex between that of the two adjacent layers or components. In someembodiments, an index matching layer includes two or more sublayers toprovide a more gradual shift in refractive index compared to using asingle layer. An antireflection coating may be a single layer (e.g., ananostructured antireflection layer) or may include two or moresublayers. For example, alternating layers of differing refractiveindices with thicknesses selected to result in destructive interferencemay be used. The index matching layer(s) and/or antireflectioncoating(s) may include one or more inorganic layers, such as layers ofalumina, titania, silica, MgF₂, for example.

Any suitable optically clear adhesives may be used for the opticallyclear adhesive layers. An optically clear adhesive is an adhesive with asuitably high transmittance in the predetermined wavelength range (e.g.,a transmittance of at least 80%, or at least 85%, or at least 90% forsubstantially normally incident substantially unpolarized light in thepredetermined wavelength range) and a suitably low haze (e.g., a haze ofless than 20%, or less than 10%, or less than 5% as determined by theASTM D1003-13 test standard).

FIG. 8 is a schematic plot of the transmittance of an optical film, orof a plurality of polymeric layers (e.g., a plurality of interferencelayers reflecting and transmitting light primarily by opticalinterference and/or alternating first and second polymeric layers)included in the optical film, for orthogonal first and secondpolarization states for substantially normally incident light. Theaverage of the transmittance over wavelengths in the predeterminedwavelength range from 11 to λ2 is Ts in the first polarization state andTp in the second polarization state. In some embodiments, λ1 is in arange from about 400 nm to about 450 nm, and λ2 is in a range from about600 nm to about 700 nm or from about 650 nm to about 700 nm.

FIG. 9 is a schematic plot of the reflectance of an optical film, or ofa plurality of polymeric layers (e.g., a plurality of interferencelayers reflecting and transmitting light primarily by opticalinterference and/or alternating first and second polymeric layers)included in the optical film, for orthogonal first and secondpolarization states for substantially normally incident light. Theaverage of the reflectance over wavelengths in the predeterminedwavelength range from 11 to λ2 is Rs in the first polarization state andRp in the second polarization state.

The transmittance and/or reflectance shown in FIGS. 8-9 may be for alocation on an optical film and each location on the film may have acorresponding transmittance and reflectance which may vary somewhat fromlocation to location due to a forming (e.g., thermoforming) process, forexample. For example, each location may have a corresponding reflectionband generally as illustrated in FIG. 9, but the band edge wavelengthsλ0 and/or λ3 may vary somewhat with position. An optical film may besaid to have an average transmittance and/or reflectance in a specifiedrange if at least one location on the optical film has an averagetransmittance and/or reflectance in the specified range. In someembodiments, each location in at least a majority of the area of theoptical film, or each location in all or substantially all of the areaof the optical film may have the specified average transmittance and/orreflectance.

A long wavelength band edge λ3 is illustrated in FIGS. 8-9 and a shortwavelength band edge λ0 is indicated in FIG. 9. Reflection bandstypically have both long and short wavelength band edges where thereflectance rapidly drops. In the illustrated embodiment, the shortwavelength band edge λ0 is less than λ1 and the long wavelength bandedge λ3 is greater than λ2. The band edges are determined forsubstantially normally incident light. The precise wavelength of a bandedge can be defined using several different criteria. The wavelength ofthe band edge may be can be taken to be the wavelength where thereflectance for normally incident light having the first polarizationstate drops to ½ Rs or the wavelength where the transmittance fornormally incident light having the first polarization state increases to10%, for example.

The materials used in the manufacture of multilayer optical films aretypically polymer materials that have very low absorption at least overvisible and near-visible wavelengths and for typical optical pathdistances within the film. Thus, the % reflection R and the %transmission T of a multilayer film for a given light ray are typicallysubstantially complementary, i.e., R+T≈100%, usually within an accuracyof about 1%.

The transmittance of an optical film refers generally to the transmittedlight intensity divided by the incident light intensity (for light of agiven wavelength, incident direction, etc.), but may be expressed interms of “external transmittance” or “internal transmittance”. Theexternal transmittance of an optical film is the transmittance of theoptical film when immersed in air, and without making any correctionsfor Fresnel reflections at the air/element interface at the front of theelement or for Fresnel reflections at the element/air interface at theback of the element. The internal transmittance of an optical film isthe transmittance of the film when the Fresnel reflections at its frontand back surfaces have been removed. The removal of the front and backFresnel reflections may be done either computationally (e.g. bysubtracting an appropriate function from the externaltransmission/reflection spectrum which can be determined from theFresnel equations and measured refractive indices of the outermostlayers of the optical film), or experimentally (e.g., the internaltransmittance and internal reflectance can be extracted frommeasurements of reflectance and transmittance of the optical film withand without an absorptive polarizer disposed on the front sides of theoptical film and suitably aligned with the optical film, measurements ofreflectance and transmittance of the optical film with and withoutabsorptive polarizers disposed on the front and the back sides of theoptical film and suitably aligned with the optical film, andmeasurements of the reflectance and transmittance of the absorbingpolarizer). For many types of polymer and glass materials, the Fresnelreflections are about 4 to 6% (for normal or near-normal angles ofincidence) at each of the two outer surfaces, which results in adownward shift of about 10% for the external transmittance relative tothe internal transmittance.

Thus, the internal transmission of an optical film refers to thetransmission that results only from interior portions of the filmcomponent, and not the two outer surfaces thereof. Analogous to internaltransmission is “internal reflection”. The internal reflection of a filmrefers to the reflection that results only from interior portions of thefilm component, and not the two outermost surfaces thereof. Thetransmittance or reflectance of an optical film may also be determinedfor the film immersed in some medium such as a glass. For example, if aglass has a refractive index close to that of the outer layers of theoptical film and an index matched adhesive is used to bond the opticalfilm to glass (e.g., a glass prism) on each side of the optical film,the normally incident transmittance and reflectance of the optical filmimmersed in the glass will be approximately equal to the normallyincident internal transmittance and reflectance, respectively. Iftransmittance or reflectance is referred to herein without beingspecified as internal or external, it should be assumed that thetransmittance or reflectance refers to internal transmittance orinternal reflectance, respectively, unless otherwise indicated by thecontext.

The internal reflection and transmission characteristics can be readilydetermined from optical modeling or from laboratory measurements. In thecase of calculated values of reflectivity and transmission for a modeledfilm, the internal reflection and transmission is readily accomplishedby omitting the calculation of those surface reflectivities from thecomputed value. The reflection spectrum and all of its features such asthe reflectivity at any angle and the band edges for birefringentmultilayer films can be calculated using the 4×4 stack code of Berremanand Scheffer, Phys. Rev. Lett. 25, 577 (1970). A description of thismethod is given in the book “Ellipsometry and Polarized Light” writtenby Azzam and Bashara, published by Elsevier Science, Holland.

In the case of measured values of reflectivity or transmission, theinternal reflection and transmission characteristics can be determinedby taking measurements of the film in air and subtracting a calculatedor measured value representative of only the surface reflectivities. Forexample, given a multilayer film having a smooth and clear surface layerwhich is much thicker than the interference layers, the index ofrefraction of this surface layer can be measured. Once the index of thesurface layer is known, the surface reflectivity can be subtracted fromthe total measured reflectivity by using mathematical formulas which arewell known in the art.

FIG. 10A is a schematic perspective view of an optical film 3100 whichmay be a reflective polarizer and may be used in any of the opticalsystems described elsewhere herein. FIG. 10B is a schematic perspectiveview of a segment of the optical film 3100. Optical film 3100 includes aplurality of polymeric interference layers 3102 having a total of (N)interference layers. FIG. 10B illustrates alternating higher index(A-layers) and lower index (B-layers) polymeric layers 3102 a and 3102b. The higher index layers have an index in at least one directiongreater than an index of the lower index layers in the same direction.The lower index layers 3102 b may be referred to as first layers and thehigher index layers 3102 a may be referred to as second layers. In someembodiments, for a substantially normally incident light in apredetermined wavelength range extending at least from about 450 nm toabout 600 nm, a maximum index of refraction of the second polymericlayer is greater than a maximum index of refraction of the firstpolymeric layer, and a difference between the maximum index ofrefraction of the second polymeric layer and a minimum index ofrefraction of the first polymeric layer is less than about 0.3, or lessthan about 0.28, or in a range of about 0.2 to about 0.3, or in a rangeof about 0.22 to about 0.28, or is about 0.25. Since substantiallynormally incident light is referred to, the maximum and minimum indicesof refraction are substantially in-plane indices.

In some embodiments, the plurality of alternating first and secondpolymeric layers 3102 b and 3102 a include less than about 900 layers,or less than about 500 layers, or less than about 300 layers, or includea total number (N) of layers in a range of about 200 to about 300layers. In some embodiments, optical film 3100 has an average thicknesst of less than about 500 microns. The average thickness refers to thethickness average over the area of the optical film. In someembodiments, the thickness is substantially uniform so that thethickness of the optical film is substantially equal to the averagethickness t. In some embodiments, the optical film is formed into acurved shape and has a thickness variation resulting from the formingprocess. In some embodiments, each polymeric layer 3102 has an averagethickness of less than about 500 nm.

During use, light incident on a major surface of optical film 3100(e.g., film surface 3104), depicted by incident light 3110 may enter afirst layer of optical film 3100 and propagate through the plurality ofinterference layers 3102, undergoing select reflection or transmissionby optical interference depending on the polarization state of incidentlight 3110. Incident light 3110 may include a first polarization state(b) and a second polarization state (a) that are be mutually orthogonalto one another. In some embodiments, the optical film 3100 is apolarizer and the second polarization state (a) may be considered as the“pass” state while the first polarization state (b) may be considered asthe “block” state. In some embodiments, optical film 3100 is a polarizeroriented along a stretch axis 3120 and not oriented along an orthogonalaxis 3122. In such embodiments, the polarization state of normallyincident light having an electric field along the axis 3122 is thesecond polarization state (a) and the polarization state of normallyincident light having an electric field along the axis 3120 is the firstpolarization state (b). In some embodiments, as incident light 3110propagates through plurality of interference layers 3102, portions ofthe light in the first polarization state (b) is reflected by adjacentinterference layers resulting in the first polarization state (b) beingreflected by optical film 3100, while a portion of the light in thesecond polarization state (a) collectively passes through optical film3100.

Interference layers may be described as reflecting and transmittinglight primarily by optical interference when the reflectance andtransmittance of the interference layers can be reasonably described byoptical interference or reasonably accurately modeled as resulting fromoptical interference. Adjacent pairs of interference layers havingdifferent refractive indices reflect light by optical interference whenthe pair has a combined optical thickness (refractive index along theblock axis times physical thickness) of ½ the wavelength of the light.In some embodiments, the optical thicknesses of adjacent pairs ofinterference layers in an optical repeat unit are about equal.Interference layers typically have a physical thickness of less thanabout 500 nm or less than about 200 nanometers. In some embodiments,each polymeric interference layer has an average thickness (unweightedaverage of the physical thickness over the layer) in a range of about 45nanometers to about 200 nanometers. Noninterference layers have anoptical thickness too large to contribute to the reflection of visiblelight via interference. Noninterference layers typically have a physicalthickness of at least 1 micrometer, or at least 5 micrometers. Theinterference layers 3102 may be a plurality of polymeric interferencelayers reflecting and transmitting light primarily by opticalinterference in the predetermined wavelength range. The averagethickness of the optical film including the interference layers and thenoninterference layers may be less than about 500 microns.

Suitable materials for the alternating polymeric layers include, forexample, polyethylene naphthalate (PEN), copolymers containing PEN andpolyesters (e.g., polyethylene terephthalate (PET) or dibenzoic acid),glycol modified polyethylene terephthalate, polycarbonate (PC), orblends of these classes of materials.

Methods of making optical films including alternating polymericinterference layers are known in the art and are described in U.S. Pat.No. 5,882,774 (Jonza et al.), U.S. Pat. No. 6,179,948 (Merrill et al.),U.S. Pat. No. 6,783,349 (Neavin et al.), and U.S. Pat. No. 9,162,406(Neavin et al.), for example. In brief summary, the fabrication methodcan include: (a) providing at least a first and a second stream of resincorresponding to the first and second polymers to be used in thefinished film; (b) dividing the first and the second streams into aplurality of layers using a suitable feedblock; (c) passing thecomposite stream through an extrusion die to form a multilayer web inwhich each layer is generally parallel to the major surface of adjacentlayers; and (d) casting the multilayer web onto a chill roll, sometimesreferred to as a casting wheel or casting drum, to form a castmultilayer film. This cast film may have the same number of layers asthe finished film, but the layers of the cast film are typically muchthicker than those of the finished film. Furthermore, the layers of thecast film are typically all isotropic. After the multilayer web iscooled on the chill roll, it can be drawn or stretched to produce afinished or near-finished multilayer optical film. The drawing orstretching accomplishes two goals: it thins the layers to their desiredfinal thicknesses, and it orients the layers such that at least some ofthe layers become birefringent. The orientation or stretching can beaccomplished along the cross-web direction (e.g., via a tenter), alongthe down-web direction (e.g., via a length orienter), or any combinationthereof, whether simultaneously or sequentially.

In some embodiments, the reflective polarizer comprises a plurality ofalternating first and second polymeric layers, where each secondpolymeric layer is substantially uniaxially oriented (e.g., along axis3120 depicted in FIG. 10B) at one or more locations (e.g., at alllocation when the reflective polarizer is substantially planar and atlocations from an apex of a curved reflective polarizer along an arcgenerally along the block axis of the reflective polarizer). Asubstantially uniaxially oriented layer has indices of refraction in onein-plane (e.g., length) direction and a thickness direction that aresubstantially the same (e.g., within 0.02 or within 0.01), butsubstantially different (e.g., a least 0.05 different) from an index ofrefraction in an orthogonal in-plane (e.g., width) direction. In someembodiments, the multilayer optical film, prior to thermoforming, is asubstantially uniaxially drawn film and has a degree of uniaxialcharacter U of at least 0.7, or at least 0.8, or at least 0.85, whereU=(1/MDDR−1)/(TDDR^(1/2)−1) with MDDR defined as the machine directiondraw ratio and TDDR defined as the transverse direction draw ratio. Suchsubstantially uniaxially oriented multilayer optical films are describedin U.S. Pat. Appl. No. 2010/0254002 (Merrill et al.) and can be obtainedby using a parabolic tenter to orient the multilayer film. As describedin U.S. Pat. No. 9,557,568 (Ouderkirk et al.) substantially uniaxiallyoriented reflective polarizer films offer improved performance in foldedoptical systems. In some embodiments, the second layers the plurality ofalternating first and second polymeric layers have refractive indices inthe pass and thickness directions than differ by less than about 0.008(e.g., a difference greater than about 0.002 and less than about 0.008).

In some embodiments, the plurality of alternating first and secondpolymeric layers has an average optical reflectance Rs for the firstpolarization state, and an average optical transmittance Tp and anaverage optical reflectance Rp for the second polarization state, whereTp≥80%; Rp≤1^(%), or Rp≤0.8%, or Rp≤0.6%, or Rp≤0.4%, or Rp≤0.25%; and80%≤Rs≤95%. The reflectivity for the pass state (second polarizationstate) can be made desirably low by closely matching the refractiveindices in the pass direction for the first and second layers (e.g., adifference less than about 0.006 or less than about 0.005) and/or bylimiting the number of alternating first and second polymeric layers(e.g., in some embodiments, the alternating first and second polymericlayers number between 200 and 500, and in some embodiments, theplurality of alternating first and second polymeric layers include lessthan about 300 layers). The reflectivity in the block direction (firstpolarization state) can be desirably high (e.g., 80%≤Rs≤95%) when alimited number of layers are included by utilizing significantlydifferent refractive indices in the block direction for the first andsecond layers (e.g., a difference greater than about 0.2 or greater thanabout 0.22).

The refractive indices in the pass direction for the first and secondlayers can be closely matched by a suitable choice of materials and by asuitable choice of draw ratios MDDR and TDDR. In some cases, for a givenTDDR, the MDDR is reduced below what is commonly utilized in filmprocesses in order to produce a lower Rp. Utilizing such a low MDDR cancause buckling in the optical film during processing, but areas of theoptical film free of buckles and large enough to be used in the opticalsystems of the present description are produced. For example, isotropicfirst layers of 85 wt. % polycarbonate blended with 15 wt % PETG (glycolmodified polyethylene terephthalate) had refractive indices of n1x, n1yand n1z each about 1.5792 at a wavelength of about 550 nm and each about1.5705 at a wavelength of 633 nm; and oriented second layers of 90/10coPEN, a polymer composed of 90% polyethylene naphthalate (PEN) and 10%polyethylene terephthalate (PET), oriented with draw ratios of TDDR ofabout 6 and MDDR of about 0.43 had refractive indices n2x, n2y and n2zof about 1.8372, 1.5755, and 1.5690, respectively, at a wavelength ofabout 550 nm and of about 1.8120, 1.5652, and 1.5587, respectively, at awavelength of about 633 nm. This provided a reasonably close matchbetween refractive indices in the pass direction for the first andsecond layers.

In some embodiments, the reflective polarizer includes a plurality ofalternating first and second polymeric layers, where each secondpolymeric layer is substantially uniaxially oriented at one or morelocations. In some embodiments, the reflective polarizer includes atleast one layer that is substantially optically uniaxial at at least onelocation on the at least one layer away from the optical axis passingthrough an apex of the reflective polarizer. In some embodiments, the atleast one layer at the at least one location has a first refractiveindex in a thickness direction, a second refractive index in a seconddirection orthogonal to the thickness direction, and a third refractiveindex in a third direction orthogonal to the thickness direction and tothe second direction, where an absolute value of a difference in thefirst and second refractive indices is less than 0.02, or less than0.01, or less than 0.008; and an absolute value of a difference in thesecond and third refractive indices is greater than 0.05, or greaterthan 0.1, or greater than 0.15, or greater than 0.2.

The refractive indices of the first and second polymeric layers can beexpressed in terms of refractive index components along directionsdefined by orthogonal polarization states for substantially normallyincident light and/or can be expressed in terms of refractive indexcomponents along orthogonal first and second axes in a plane of theoptical film. In some embodiments, for each pair of adjacent first andsecond polymeric layers: the first layer 3102 b has an index n1x alongthe first polarization state (e.g., polarized along axis 3120 or alongthe x-axis), an index of refraction n1y along the second polarizationstate (e.g., polarized along axis 3123 or along the y-axis), and anindex n1z along a z-axis orthogonal to the first and second polarizationstates; and the second layer 3102 a has an index n2x along the firstpolarization state, an index of refraction n2y along the secondpolarization state, and an index n2z along the z-axis. In someembodiments, for each pair of adjacent first and second polymericlayers: the first layer 3102 b has an index n1x along a first axis in aplane of the optical film (e.g., axis 3120 or the x-axis), an index ofrefraction n1y along an orthogonal second axis in the plane of theoptical film (e.g., axis 3123 or the y-axis), and an index n1z along az-axis orthogonal to the first and second axes; and the second layer3102 a has an index n2x along the first axis, an index of refraction n2yalong the second axis, and an index n2z along the z-axis. The plane ofthe optical film generally refers to a plane tangent to the opticalfilm. If the optical film is disposed in a plane, this plane is theplane of the optical film. If the optical film is curved (e.g., disposedon and conforming to a curved surface of a lens), the plane of theoptical film is the tangent plane at the location where the refractiveindices are specified. Substantially normally incident light may bepolarized in a first polarization state along the first axis (i.e., thefirst polarization state may be a linear polarization state having theelectric field along the first axis) or in a second polarization statepolarization state along the second axis (i.e., the second polarizationstate may be a linear polarization state having the electric field alongthe second axis), for example, and the optical film may have areflectance and transmittance for the first and second polarizationstates in any of the ranges described elsewhere herein.

In some embodiments, for at least one wavelength in a predeterminedwavelength range extending at least from about 450 nm to about 600 nm: amaximum difference between n1x, n1y and n1z is less than about 0.002;and a difference between n2x and n1x is greater than about 0.2. In someembodiments, for the at least one wavelength in the predeterminedwavelength range, a difference between n2y and n2z is greater than about0.002 and less than about 0.008. In some embodiments, for the at leastone wavelength in the predetermined wavelength range, an absolute valueof a difference between n1y and n2y is less than about 0.006, or lessthan about 0.005. In some embodiments, for the at least one wavelengthin the predetermined wavelength range, a difference between n2x and n1xis in a range of about 0.22 to about 0.28. The indices of refraction canbe adjusted by a suitable selection of polymers for the first and secondlayers any by a suitable degree of orientation of the layers (e.g.,increasing draw ratio can increase the refractive index in the stretchdirection in the oriented (higher index) layers). The indices ofrefraction may be specified for at least one wavelength in thepredetermined wavelength range, for example, and/or may be specified ata fixed reference wavelength (e.g., 532 nm or 550 nm or 633 nm). Forexample, in some embodiments, at a wavelength of about 550 nm, n1x, n1yand n1z are each about 1.5792 and n2x, n2y and n2z are about 1.8372,1.5755, and 1.5690, respectively. In some embodiments, at a wavelengthof about 633 nm, n1x, n1y and n1z are each about 1.5705 and n2x, n2y andn2z are about 1.8120, 1.5652, and 1.5587, respectively. In someembodiments, the optical film has refractive indices in any of theseranges at at least one location on the optical film. In someembodiments, the optical film has refractive indices in any of theseranges over at least 80%, or at least 90%, or substantially all, of atotal area of the optical film.

In some embodiments, the optical system includes a retarder disposedbetween the partial reflector and the reflective polarizer. In someembodiments, the retarder disposed between the partial reflector and thereflective polarizer has a quarter wave retardance at one or morewavelengths in the predetermined wavelength range. In some embodiments,the optical system includes a plurality of retarders (e.g., theplurality of retarders depicted in FIG. 6 or in FIG. 7). In someembodiments, each retarder has a quarter wave retardance at one or morewavelengths in the predetermined wavelength range.

The retarder layer(s) used in the optical systems of the presentdescription can be films or coatings or a combination of films andcoating. Suitable films include birefringent polymer film retarders suchas those available from Meadowlark Optics (Frederick, Colo.), forexample. Suitable coatings for forming a retarder layer include thelinear photopolymerizable polymer (LPP) materials and the liquid crystalpolymer (LCP) materials described in U.S. Pat. App. Pub. Nos.2002/0180916 (Schadt et al.), 2003/028048 (Cherkaoui et al.),2005/0072959 (Moia et al.) and 2006/0197068 (Schadt et al.), and in U.S.Pat. No. 6,300,991 (Schadt et al.). Suitable LPP materials includeROP-131 EXP 306 LPP and suitable LCP materials include ROF-5185 EXP 410LCP, both available from ROLIC Technologies Ltd. (Allschwil,Switzerland).

FIG. 11A is a schematic illustration of retardance (e.g., in nm) versuswavelength for a retarder. The retardance versus wavelength is awavelength dispersion curve 1600 a. The dispersion curve 1600 a issimilar to the dispersion curve of an achromatic retarder available fromMerck (Darmstadt, Germany), for example. The dispersion curve 1607 of anideal achromatic quarter-wave retarder is also shown. The retarderhaving the dispersion curve 1600 a may be a quarter-wave retarder at thewavelength λa and at λb, for example, and may be substantiallyquarter-wave (e.g., a retardance within 5% of quarter wave) over awavelength range in the predetermined wavelength range (e.g., from λ1 toλ2). FIG. 11B is a schematic illustration of a wavelength dispersioncurve 1600 b for a different retarder layer. The dispersion curve 1600 bis similar to a dispersion curve of a conventional retarder such as thatof an oriented polymer layer. The dispersion curve 1600 b changesmonotonically with increasing wavelength in the predetermined wavelengthrange. The retarder layer having the dispersion curve 1600 b may be asubstantially quarter-wave retarder at the wavelength λ3, for example.

Any of the optical systems or display systems of the present descriptionmay be used in a device such as a headset (e.g., a headset for ahead-mounted display or a virtual or augmented reality display). Theheadset may include one or more display panels which may be removable(e.g., the headset may be adapted to receive a phone which has a displaypanel). FIG. 19 is a schematic top view of headset 1790 including aframe 1792, and left and right display portions 1794 a and 1794 b whichmay include respective left and right optical systems where each of theleft and right optical systems is an optical system according to thepresent description. An optional removable display panel 1785 isillustrated. In other embodiments, left and right display panels or leftand right portions of a display panel are included in the left and rightdisplay portions 1794 a and 1794 b.

If the use of “about” as applied to quantities expressing feature sizes,amounts, and physical properties is not otherwise clear to one ofordinary skill in the art in the context in which it is used anddescribed in the present description, “about” will be understood to meanwithin 10 percent of the specified quantity, but also includes exactlythe specified quantity. For example, if it is not otherwise clear to oneof ordinary skill in the art in the context in which it is used anddescribed in the present description, a quantity having a value of about1, means that the quantity has a value between 0.9 and 1.1, but alsoincludes a value of exactly 1.

EXAMPLES Example 1

A reflective polarizer was made as follows. A single multilayer opticalpacket was co-extruded. The packet included 275 alternating layers of90/10 coPEN and low index isotropic layers. 90/10 coPEN is a polymercomposed of 90% polyethylene naphthalate (PEN) and 10% polyethyleneterephthalate (PET). The isotropic layers were made with a blend ofpolycarbonate and copolyesters (PC:coPET) as described in WO2015035030such that the index was about 1.57 and the layers remained substantiallyisotropic upon uniaxial orientation. The PC:coPET molar ratio wasapproximately 42.5 mol % PC and 57.5 mol % coPET. The isotropic layershad a Tg of 105 degrees centigrade. This isotropic material was chosensuch that after stretching its refractive indices in the two non-stretchdirections remained substantially matched with those of the birefringentmaterial in the non-stretching direction while in the stretchingdirection there was a substantial mismatch in refractive indices betweenbirefringent and non-birefringent layers. The 90/10 PEN and PC:coPETpolymers were fed from separate extruders ratios of total flow of 44%and 56% for 90/10 coPEN and PC:coPET respectively to a multilayercoextrusion feedblock. The materials were assembled into a packet of 275alternating optical layers, plus a thicker protective boundary layer ofthe on each side of 90/10 PEN on one side and PC:coPET on the other, fora total of 277 layers. The multilayer melt was then cast through a filmdie onto a chill roll, in the conventional manner for polyester films,upon which it was quenched. The cast web was then stretched in aparabolic tenter similar to that described in the Invited Paper 45.1,authored by Denker et al., entitled “Advanced Polarizer Film forImproved Performance of Liquid Crystal Displays,” presented at Societyfor Information Displays (SID) International Conference in SanFrancisco, Calif., Jun. 4-9, 2006. The transverse direction draw ratio(TDDR) in the parabolic tenter was about 6.0 and the machine directiondraw ratio (MDDR) was about 0.43.

The reflectance and transmittance in air (external reflectance andtransmittance) at normal incidence was measured using a LAMBDA 1050UV/Vis Spectophotometer with polarizer option available from PerkinElmer(Waltham, Mass.). The CIE Illuminant C was used for the light source.The resulting transmittance and reflectance in air for the reflectivepolarizer of Example 1 and for a sample of Advanced Polarizing Film(APF) available from 3M Company (St. Paul, Minn.) are shown in FIGS.13-14. Next, the refractive indices of an outermost layer of thereflective polarizer was measured with a Metricon 2010/M prism couplerat wavelengths of 404 nm, 532 nm, and 633 nm. The table below lists themeasured refractive indices of the outermost layers (each outermostlayer of the reflective polarizer used a same polymer material and sothe refractive indices were assumed to the be same in each outermostlayer) in the machine direction (MD) corresponding to the pass axis, thetransverse direction (TD) corresponding to the block axis, and thethickness (Z) direction for the reflective polarizer of Example 1 andfor a sample of APF.

633 nm 532 nm 404 nm MD TD Z MD TD Z MD TD Z APF 1.5745 1.5676 1.56881.5797 1.5799 1.5786 1.6093 1.6111 1.6111 Example 1 1.5708 1.5733 1.57271.5809 1.5838 1.5804 1.6100 1.6137 1.6096

From the measured refractive indices, the photopically averagedreflectance of each of the surfaces was determined to be about 5.03percent using Macleod optical modeling software available from Thin FilmCenter Inc (Tucson, Ariz.). The internal reflectance and internaltransmittance was then determined by simultaneously solving theequations describing the external reflectance and transmittance in termsof the internal reflectance and transmittance and in terms of repeatedFresnel reflections from the front and rear surfaces. The series ofrepeated reflections included up to 5 reflections from the reflectivepolarizer. The photopically averaged results for the reflectivepolarizer of Example 1 were R_(s) ^(ph)=92.4%, R_(p) ^(ph)=0.15%, T_(s)^(ph)=7.8 and T_(p) ^(ph)=99.7%. All photopic averaging was carried outbased on the CIE 1931 2° Standard Observer. In comparison, the resultsof the measurement method applied to a sample of APF were R_(s)^(ph)=98.8%, R_(p) ^(ph)=1.20, T_(s) ^(ph)=1.2%, and T_(p) ^(ph) 98.4%.

An optical system similar to optical system 600 was constructed where asample of the reflective polarizer of Example 1 was used as thereflective polarizer 630. The optical system utilized a checkerboardpattern on glass available from Precision Optical Imaging (Rush, N.Y.)in place of the display panel 604. Each of the optically clear adhesivelayers used in the optical system was a layer of Optically ClearAdhesive 8172 available from 3M Company (St. Paul, Minn.). The retarders685, 625, and 663 were each a Teijin FM143 quarter-wave retarderavailable from Teijin Co. (Japan). For the retarders 685 and 625, anindex matching layer consisting essentially of a SiO₂ layer disposeddirectly on the retarder and an Al₂O₃ layer disposed on the SiO₂ layerwas included on each major surface of the retarder. The antireflectionlayer 629 was replaced with a similar index matching layer and theabsorptive polarizer 661 was laminated to this index matching layer andthe retarder 663 was laminated to the absorptive polarizer 661 throughanother optically clear adhesive layer. An index matching layer asdescribed above was disposed on the retarder 663 adjacent this opticallyclear adhesive layer. An antireflection coating was disposed on theretarder 663 facing the exit surface 652. This antireflection coatingand the antireflection coating 649 consisted essentially of alternatinglayers of TiO₂ and MgF₂ for a total of 6 layers. The absorptivepolarizer 691 was an AP38 polarizer available from API AmericanPolarizers, Inc. (Reading, Pa.). The absorptive polarizer 661 was an APSanritz HLC2-5618 available from Sanritz Co. (Japan). The partialreflector 620 was a half-silvered mirror. The lens 610 was aplano-convex glass lens available from Edmund Optics (Barrington, N.J.).

A 14×14 array of squares arranged in checkerboard pattern of bright anddark squares within about a 19 mm aperture was displayed by using awhite light emitting diode (LED) light source to provide light to theglass having the checkerboard pattern. An image was formed from thedisplayed pattern and a square region of 4 squares by 4 squares (4squares across included 2 bright and 2 dark squares) near the center ofthe image was analyzed. The intensity in the bright and dark squares wasmeasured with a color camera which recorded a number of counts in eachof a red, green, and blue color channel that was proportional to theintensity. A lens and diffuser was placed between the LED and the glasshaving the checkerboard pattern to provide a full cone angle of about 30degrees. Masks were defined to exclude transition regions from thebright and dark regions (squares). This defined a central region of eachbright and dark region. The area of the central regions was varied andfound to have minimal effect on the resulting Ib/Id ratio when the areaof the central regions was in a range of about 30% to about 70% of thearea of the corresponding bright or dark region. Camera exposures of 8ms, 12 ms, 16 ms, and 24 ms were used and measurements were repeatedwith the camera lens cap in place to dark subtract in order to providethe correct baseline. The results of the different exposures wereintegrated to form a high dynamic range image. The average (mean)brightness Ib in the central regions of the bright regions wasdetermined and the average (mean) brightness Id in the central regionsof the dark regions was determined for each of the red, green, and bluecolor channels. Another optical system was prepared that was equivalentto the tested optical system except that the reflective polarizer wasreplaced with APF. The ratios Ib/Id was determined for this opticalsystem as described above. The results are provided in the followingtable.

Ib/Id Color Channel Example 1 Optical System with APF Red 66.7 51.7Green 82.6 69.0 Blue 76.6 60.0

Example 2

A reflective polarizer similar to the reflective polarizer of Example 1was modeled. The 4×4 matrix method using the Berreman algorithm was usedto determine the spectra of constructive and destructive interferencegenerated from layer interfaces in the reflective polarizer. Thereflective polarizer was modeled as having 275 alternating birefringentand isotropic layers where the thickness and the refractive indices ofthe birefringent and isotropic layers were modeled based on propertiesof the birefringent and isotropic layers of the reflective polarizer ofExample 1. The refractive indices of the isotropic layer in eachdirection were taken to be 1.5792 at a wavelength of 550 nm and 1.5705at a wavelength of 633 nm, and smoothly varying between and above 633 nmwavelength, and blow 500 nm wavelength. The refractive index of thebirefringent layers in the block direction was taken to be 1.8372 at 550nm and 1.8120 and 633 nm, similarly varying across the visible spectrum.The refractive index of the birefringent layers in the pass directionwas taken to be 1.5755 at 550 nm and 1.5652 and 633 nm. The refractiveindex of the birefringent layers in the thickness direction was taken tobe 1.5690 at 550 nm and 1.5587 and 633 nm. Both were taken as smoothlyvarying across the visible spectrum, as above. Optical stack thicknessfor the reflective polarizer was obtained by an Atomic Force Microscopymeasurement. The internal reflectance for the pass and block states forlight normally incident on the reflective polarizer optical stack wascalculated, by embedding the entire stack in ½ spaces of material whoserefractive indices matched those of the exterior layers on thereflective polarizer stack. The results averaged over wavelengths from450 nm to 650 nm were Rs=89.0% and Rp=0.198%.

All references, patents, and patent applications referenced in theforegoing are hereby incorporated herein by reference in their entiretyin a consistent manner. In the event of inconsistencies orcontradictions between portions of the incorporated references and thisapplication, the information in the preceding description shall control.

Descriptions for elements in figures should be understood to applyequally to corresponding elements in other figures, unless indicatedotherwise. Although specific embodiments have been illustrated anddescribed herein, it will be appreciated by those of ordinary skill inthe art that a variety of alternate and/or equivalent implementationscan be substituted for the specific embodiments shown and describedwithout departing from the scope of the present disclosure. Thisapplication is intended to cover any adaptations or variations of thespecific embodiments discussed herein. Therefore, it is intended thatthis disclosure be limited only by the claims and the equivalentsthereof.

1. An optical system comprising: one or more optical lenses having atleast one curved major surface; a partial reflector; and a reflectivepolarizer, such that for a substantially normally incident light in apredetermined wavelength range extending at least from about 450 nm toabout 600 nm: the partial reflector has an average optical reflectanceof at least 30%; and the reflective polarizer has an average opticalreflectance Rs for a first polarization state, an average opticaltransmittance Tp for an orthogonal second polarization state, and anaverage optical reflectance Rp for the second polarization state,Tp≥80%, Rp≤1%, and 50%≤Rs≤95%.
 2. The optical system of claim 1, whereinthe reflective polarizer comprises a plurality of interference layersreflecting and transmitting light primarily by optical interference,such that for a substantially normally incident light in thepredetermined wavelength range, the plurality of interference layers hasan average optical reflectance for the first polarization state in arange of 50% to 95%, an average optical transmittance for the secondpolarization state of at least 80%, and an average optical reflectancefor the second polarization state of no more than 1%.
 3. The opticalsystem of claim 1, wherein for a substantially normally incident lightin the predetermined wavelength range, the reflective polarizer has anaverage optical transmittance Ts of less than about 10%.
 4. The opticalsystem of claim 1 being configured to display an image to a viewerpositioned proximate an exit surface of the optical system, wherein foran incident cone of light having a full cone angle of at least 10degrees that is incident on the optical system from an object comprisinga spatial frequency of less than about 1 line pair per millimeter, andthat exits the optical system through the exit surface as an exitingcone of light, when the exiting cone of light is imaged proximate theexit surface, the image has a plurality of alternating bright and darkregions, Ib being an average brightness of a central 90% of the brightregions, Id being an average brightness of a central 90% of the darkregions, Ib/Id≥50.
 5. The optical system of claim 1, wherein thereflective polarizer comprises a plurality of alternating first andsecond polymeric layers, each polymeric layer having an averagethickness of less than about 500 nm, such that for a substantiallynormally incident light in a predetermined wavelength range extending atleast from about 450 nm to about 600 nm: a maximum index of refractionof the second polymeric layer is greater than a maximum index ofrefraction of the first polymeric layer, and a difference between themaximum index of refraction of the second polymeric layer and a minimumindex of refraction of the first polymeric layer is less than about 0.3.6. The optical system of claim 1, wherein the reflective polarizercomprises a plurality of alternating first and second polymeric layers,each first and second polymeric layer having an average thickness lessthan about 500 nm, for each pair of adjacent first and second polymericlayers: the first layer has an index n1x along the first polarizationstate, an index of refraction n1y along the second polarization state,and an index n1z along a z-axis orthogonal to the first and secondpolarization states; and the second layer has an index n2x along thefirst polarization state, an index of refraction n2y along the secondpolarization state, and an index n2z along the z-axis, such that for atleast one wavelength in a predetermined wavelength range extending atleast from about 450 nm to about 600 nm: a maximum difference betweenn1x, n1y and n1z is less than about 0.002; and a difference between n2xand n1x is greater than about 0.2.
 7. An optical system comprising: oneor more optical lenses; a partial reflector disposed on and conformingto a curved major surface of the one or more optical lenses; areflective polarizer disposed on and conforming to a major surface ofthe one or more optical lenses and comprising a plurality of polymericlayers, each polymeric layer having an average thickness of less thanabout 500 nm, such that for a substantially normally incident light in apredetermined wavelength range extending at least from about 450 nm toabout 600 nm: the partial reflector has an average optical reflectanceof at least 30%; and the plurality of polymeric layers has an averageoptical reflectance Rs for a first polarization state, 50%≤Rs≤95%, andan average optical transmittance Tp≥80% for an orthogonal secondpolarization state; and an exit surface, the optical system configuredto display an image to a viewer positioned proximate the exit surface;such that for an incident cone of light having a full cone angle of atleast 10 degrees that is incident on the optical system from an objectcomprising a spatial frequency of less than about 1 line pair permillimeter, and exits the optical system through the exit surface as anexiting cone of light, when the exiting cone of light is imagedproximate the exit surface, the image has a plurality of alternatingbright and dark regions, Ib being an average brightness of central 50%regions of the bright regions, Id being an average brightness of central50% regions of the dark regions, Ib/Id≥50.
 8. The optical system ofclaim 7, wherein each central 50% region of the bright regions is aninterior region of a bright region having an area of about 50% of anarea of the bright region, and each central 50% region of the darkregions is an interior region of a dark region having an area of about50% of an area of the dark region.
 9. The optical system of claim 7,wherein the incident cone of light comprises wavelengths from at least520 nm to 570 nm and Ib/Id≥72.
 10. The optical system of claim 7,wherein the plurality of polymeric layers comprises a plurality ofalternating first and second polymeric layers, for each pair of adjacentfirst and second polymeric layers: the first layer has an index n1xalong the first polarization state, an index of refraction n1y along thesecond polarization state, and an index n1z along a z-axis orthogonal tothe first and second polarization states; and the second layer has anindex n2x along the first polarization state, an index of refraction n2yalong the second polarization state, and an index n2z along the z-axis,such that for at least one wavelength in a predetermined wavelengthrange extending at least from about 450 nm to about 600 nm: a maximumdifference between n1x, n1y and n1z is less than about 0.002; and adifference between n2x and n1x is greater than about 0.2.
 11. An opticalfilm comprising a plurality of alternating first and second polymericlayers numbering between 200 and 500, each first and second polymericlayer having an average thickness less than about 500 nm, for each pairof adjacent first and second polymeric layers: the first layer has anindex n1x along a first axis in a plane of the optical film, an index ofrefraction n1y along an orthogonal second axis in the plane of theoptical film, and an index n1z along a z-axis orthogonal to the firstand second axes; and the second layer has an index n2x along the firstaxis, an index of refraction n2y along the second axis, and an index n2zalong the z-axis, such that for at least one wavelength in apredetermined wavelength range extending at least from about 450 nm toabout 600 nm: a maximum difference between n1x, n1y and n1z is less thanabout 0.002; and a difference between n2x and n1x is greater than about0.2; such that for a substantially normally incident light having the atleast one wavelength in the predetermined wavelength range, theplurality of alternating first and second polymeric layers has anaverage optical reflectance Rs for a first polarization state along thefirst axis, and an average optical transmittance Tp and an averageoptical reflectance Rp for a second polarization state along the secondaxis, Tp≥80%, Rp≤0.25%, and 80%≤Rs≤95%.
 12. The optical film of claim11, such that for the at least one wavelength in the predeterminedwavelength range, a difference between n2y and n2z is greater than about0.002 and less than about 0.008.
 13. The optical film of claim 11,wherein for the at least one wavelength in the predetermined wavelengthrange, a difference between n2x and n1x is in a range of about 0.22 toabout 0.28.
 14. An optical system comprising: one or more opticallenses; a partial reflector disposed on and conforming to a curved majorsurface of the one or more optical lenses; a reflective polarizerdisposed on and conforming to a major surface of the one or more opticallenses and comprising a plurality of alternating first and secondpolymeric layers, each polymeric layer having an average thickness ofless than about 500 nm, such that for a substantially normally incidentlight in a predetermined wavelength range extending at least from about450 nm to about 600 nm: the partial reflector has an average opticalreflectance of at least 30%; and a maximum index of refraction of thesecond polymeric layer is greater than a maximum index of refraction ofthe first polymeric layer, and a difference between the maximum index ofrefraction of the second polymeric layer and a minimum index ofrefraction of the first polymeric layer and is less than about 0.3; andan exit surface, the optical system configured to display an image to aviewer positioned proximate the exit surface; such that for an incidentcone of light having a full cone angle of at least 10 degrees that isincident on the optical system from an object comprising a spatialfrequency of less than about 1 line pair per millimeter, and exits theoptical system through the exit surface as an exiting cone of light,when the exiting cone of light is imaged proximate the exit surface, theimage has a plurality of alternating bright and dark regions, Ib beingan average brightness of central 50% regions of the bright regions, Idbeing an average brightness of central 50% regions of the dark regions,Ib/Id≥50.
 15. The optical system of claim 14, wherein the differencebetween the maximum index of refraction of the second polymeric layerand the minimum index of refraction of the first polymeric layer is lessthan about 0.28.